text: The variation in the / ratio in different parts of the carbon exchange reservoir means that a straightforward calculation of the age of a sample based on the amount of  it contains will often give an incorrect result. There are several other possible sources of error that need to be considered. The errors are of four general types:
  * variations in the / ratio in the atmosphere, both geographically and over time;
  * isotopic fractionation;
  * variations in the / ratio in different parts of the reservoir;
  * contamination.